In general, energy resulting from the combustion of an air-fuel mixture when an internal combustion engine (hereinafter, also referred to as an “engine”) such as a diesel engine is in operation inevitably leads to losses, without being fully converted into work rotating a crankshaft. These losses include a cooling loss that is converted into a rise in engine main body and cooling water temperatures, an exhaust loss that is released to the atmosphere by exhaust gas, a pump loss that results during air intake and exhaust, and a mechanical resistance loss. The cooling loss and the exhaust loss account for large portions of the entire loss. Accordingly, it is effective to decrease the cooling loss and the exhaust loss when the fuel consumption rate of the engine is to be improved.
However, the cooling loss and the exhaust loss have a trade-off relationship in general, and thus it is difficult to reduce the cooling loss and the exhaust loss at the same time in many cases. In a case where the engine is provided with a turbocharger, for example, the exhaust loss is reduced because the energy contained in the exhaust gas is effectively used as a turbocharging pressure is increased. However, an actual improvement in compression rate causes a combustion temperature to increase, and thus the cooling loss increases. Accordingly, the total amount of the losses increases depending on cases.
A control device that controls a combustion state of fuel supplied to the engine (hereinafter, simply referred to as a “combustion state of the engine” in some cases) so as to reduce the total amount of the losses is required to appropriately control various parameters changing the combustion state, including a fuel injection quantity, a fuel injection timing, and the amount of EGR gas as well as the turbocharging pressure, in accordance with an operation state (rotational speed, output, or the like) of the engine. The parameters changing the combustion state of the engine (that is, the parameters affecting the combustion state of the engine) are simply referred to as “combustion parameters” in some cases. However, it is difficult to have a plurality of the combustion parameters obtained in advance by an experiment or the like as values optimal for the respective operation states, and a large-scale experiment needs to be carried out in order to determine these combustion parameters. Accordingly, techniques for systematically determining the combustion parameters have been developed.
For example, a combustion control device for an internal combustion engine according to the related art (hereinafter, also referred to as a “conventional device”) calculates a “crank angle at a point in time when half of the total amount of heat resulting during a combustion stroke is generated (hereinafter, also referred to as the “angle of the combustion center of gravity”)”. In a case where the angle of the combustion center of gravity and a predetermined reference value deviate from each other, the conventional device causes the angle of the combustion center of gravity to correspond to the reference value by correcting the fuel injection timing or adjusting an EGR rate (the amount of the EGR gas) and adjusting the oxygen concentration in a combustion chamber (in a cylinder) (for example, refer to PTL 1).